Superconducting Radio-Frequency (SRF) accelerating cavities are commonly used in accelerators. Due to their very small RF losses, much higher acceleration efficiencies, and higher continuous wave (CW) accelerating fields than normal conducting cavities, SRF cavities are now considered the device of choice for many of today's leading applications in high energy and nuclear physics, including energy recovery linacs (ERLs), linear colliders, neutrino factories, spallation neutron sources, and rare isotope accelerators. These projects place enormous demands not only on advances in beam performance, but also on more reliable and economic methods for fabrication, assembly, and operation.
The mechanical stability of an SRF cavity is a fundamental consideration in its design. Conventional SRF cavities, as a result of being constructed from metal plates, typically include a uniform wall thickness. During operation, SRF cavity walls are subject to number of forces. Of particular importance are deformations due to the Lorentz radiation pressure acting on the cavity walls, the liquid helium pressure, and the atmospheric pressure after pump down. These deformations can shift the resonant frequency, reduce field flatness, and lead to an unwanted stress distribution in the cavity. Lorentz force deformation can be a particularly difficult problem to address in the case of cavities with low β values, which have steep and flat-sided walls. The radiation pressure acting on the cavity is a function of the surface electric and magnetic fields. The Lorentz pressures are not uniformly distributed on the cavity walls as can be seen in FIG. 1.
In areas near the iris, where the electric field is highest, the Lorentz pressure acts inward, while in areas near the outer diameter, where magnetic fields dominate, it acts outward. A common practice to mitigate this problem is to use stiffener rings and other stabilizing structures welded on to the cavities. These stabilizing structures, however, introduce variability, add complexity, and cannot always be ideally placed.
Currently the construction of SRF accelerating cavities requires the use of many complex and expensive techniques to fabricate, assemble, and operate. SRF accelerating cavities are the technology of choice for many of the leading accelerator facilities in the U.S. and abroad. These projects, more than ever, are placing enormous demands on the development of more reliable and economic methods for fabricating SRF accelerating cavities and ancillary components.
Thus, the development of less-expensive, more reliable superconducting RF structures is highly desirable in the construction of SRF cavities. What is needed is a method for manufacturing SRF cavities wherein the wall thickness is designed to better withstand the Lorentz pressures, i.e., the wall of the SRF cavity is made thicker in those areas that are subject to higher Lorentz pressures and thinner in in those areas that are subject to lower Lorentz pressures. A method for constructing an SRF cavity in this manner will dramatically mitigate the effect of microphonics, and provide a cavity with an RF optimized geometry including highly tailored wall thicknesses, optimized to counteract Lorentz force detuning. The method will enable the incorporation of SRF cavities with mechanically optimized wall thicknesses, and the inclusion of stiffening structures integral with the SRF cavity.